Make the Raised Bed
Place the 8-foot-long cedar four-by-four on a flat working surface; mark a location along the post every 24 inches. Use a circular saw to cut the post along the marks so that you have four 24-inch-long posts. These are the corner posts for the raised bed.
Mark the center of two one-by-12s measuring 8 feet long. Cut the boards in half to form four 4-foot boards. These will be the short sides of the raised bed.
Place two of the corner posts made in Step 1 on a flat surface about 4 feet apart. Place a short side board from Step 2 across the two posts. Slide it down so it is flush with the bottom of the posts and screw in place with three 3-inch exterior-grade wood screws evenly spaced on each end. Attach a second short board directly above the first, leaving no gap in between. Repeat with the remaining short boards and posts to make a second short side.
Place the two short sides upright, facing each other and about 8 feet apart. Place an 8-foot-long one-by-12 across one of the posts on each side piece. Screw the board in place on the bottom of the posts with three 3-inch exterior-grade wood screws evenly spaced on each side. Attach a second 8-foot board directly above the first. Repeat on the other side of the raised bed to add two more long boards.
Make a Trellis
Place two four-by-fours measuring 6 feet long on a flat working surface. Make a mark 20 inches from the bottom on each post. Make three more marks, spaced 18 inches apart. Use a hammer to pound a fencing staple halfway into the posts at each mark. These posts will support a wire trellis.
Dig holes in the raised bed to install the trellis posts. Use post-hole diggers to make two holes 7 feet apart in the center of the raised bed. Shovel 4 inches of gravel into the bottom of the holes to help with drainage.
Place both four-by-fours, with the staples facing to the front, into the holes. Using a shovel, backfill the holes with dirt and tamp it down well.
Cut four pieces of 12-gauge fencing wire measuring 8 feet long each. Thread the first piece through the bottom two fencing staples on the trellis posts. Stretch the wire tight and pound the staples in the rest of the way to hold it in place. Repeat to install three more rows of wire.
Prepare the Bed and Plant
Spread a 1-inch layer of gravel in the bottom of the raised bed to assist with drainage.
Dump eight bags of planting soil and four bags of compost into the raised bed. Mix the soil and compost evenly inside the bed.
Plant eight raspberry plants evenly spaced on each side of the trellis. As the plants grow, tie long canes to the trellis wire. Add two inches of mulch to help with water retention and weed control.